Closed-loop purification process of industrial phosphoric acid: Selective recovery of heavy metals and rare earth elements via solvent extraction

Abstract

Phosphate fertilizers are produced using phosphoric acid as a vital chemical component. This essential ingredient is typically derived from phosphate rock, through a wet process phosphoric acid (WPPA). However, the WPPA contains impurities, including heavy metals (HMs) like cadmium (Cd), zinc (Zn) and copper (Cu), as well as unwanted elements like iron, and other valuables elements, notably rare earth elements (REEs) such as yttrium (Y), holmium (Ho) and ytterbium (Yb). The current study aims to develop a novel process for a simultaneous and selective recovery of HMs and REEs through solvent extraction from WPPA. Single-stage extraction, cascade extraction and synergistic extraction were explored to gain a deeper understanding of the mechanisms and identify the most efficient process. 90, 80 and 99 % for Y, Ho and Yb, respectively, were extracted using D2EHPA. While 89 % for Zn, 99 % for Cu and Cd were removed using Cyanex 301. On the other hand, 60 % of Yb, 74 % of Y and 78 % of Ho were stripped using H₂SO₄, while 98 % of Cd and 99 % of Zn were stripped using HCl. The organic phase can be successfully reused with an effective extraction and stripping. Oxalic acid (C₂H₂O₄) was employed to precipitate REEs. However, sodium carbonate (Na₂CO₃) was used to precipitate HMs. Finally, solvent extraction was carried out on WPPA solution to ensure the extraction performance under industrial conditions.